Feedstocks containing mixtures of phenols (such as phenol, cresols, ethylphenols, xylenols and C3 phenols) and neutral oils and/or tar bases are obtained from various sources such as petroleum and coal processing facilities.
The separation of phenols from tar bases and especially, neutral oils have always posed a challenge since certain neutral oils have boiling points close to that of the phenols and due to the fact that azeotropes are formed between the neutral oils and phenols. Normal distillation processes are accordingly not suitable to separate phenols from neutral oils and/or tar bases to the required purity.
Liquid-liquid extraction is a known method of separating phenols from neutral oils and/or tar bases. The prior art discloses that both single solvent and dual solvent processes have been employed to achieve such a separation. Dual solvent processes are preferred. In these processes a solvent is used to extract the phenols and a counter-solvent is used to extract the neutral oils. The solvent and counter-solvent are selected to be immiscible and after extraction the solvent and counter-solvent layers are separated and the phenols are recovered from the solvent layer while the neutral oils are recovered from the counter-solvent layer.
The solvent usually is a polar solvent and the counter-solvent usually is a non-polar solvent. The counter-solvent often comprises paraffins such as hexane, heptane, petroleum ether, octane and diesel. The polar solvent often comprises an aqueous solution of methanol, ammonia, acetamide, acetic acid, ethanol and glycols. In this regard see for example U.S. Pat. No. 2,666,796 where aqueous methanol and hexane were used as solvent and counter-solvent respectively. In the Journal of Applied Chemistry, Jun. 21, 1952, p. 314, and the Journal of Applied Chemistry, Mar. 3, 1953, p. 98, aqueous triethylene glycol was used as solvent. In Fuel processing Technology, 28 (1991) p. 287 aqueous solutions of ethylene glycol, diethylene glycol and triethylene glycol were used as solvents. In South African patents 98/11312 and 2001/9287 aqueous solutions of glycols were used as solvents and they were used in combination with counter-solvents.
In cases where the solvent comprises a mixture of water and a solvent compound (e.g. glycols) a solvent layer is obtained which contains the water, the solvent compound (e.g. glycols), phenols, some neutral oils and /or tar bases and some counter-solvent. The first step is usually to subject the mixture to a water recovery distillation process to strip the water from this mixture. The bottoms product consists mainly of the solvent compound (e.g. glycols) and phenols, which bottoms product is then subjected to distillation to separate the phenols from the glycols.
In South African patent 2001/9287 the solvent comprised a mixture of glycol and water and the water recovery distillation process was carried out under vacuum to prevent thermal decomposition of the glycol. The overheads product of the water recovery distillation process (which was now established to consist of water, some counter-solvent, some phenols and some neutral oils and/or tar bases) was condensed and was then mixed with counter-solvent to extract the condensate with the counter-solvent. A decanter was then used to achieve separation of the aqueous and organic layers which formed. It was suggested that the counter-solvent added resulted in extraction of organic material into the counter-solvent layer and it also resulted in better phase separation in the decanter.
One disadvantage of the above system is that the water recovery distillation process is carried out under vacuum. Under such conditions light-boiling valuables such as counter-solvent which enters the water recovery distillation process can be lost to the vacuum system. This is especially problematic where a substantial volume of counter-solvent is present in the water recovery distillation process.
Another disadvantage of this process is that a mixer apparatus is required in order to obtain proper mixing of the condensate of the water recovery distillation process with the introduced counter-solvent to cause proper extraction of the said overheads product.
It is an object of the present invention to overcome or reduce at least some of the above disadvantages.
Furthermore, South African patent 98/11312 discloses a similar water recovery step to that disclosed in South African patent 2001/9287 except that no counter-solvent was added to the overheads product of the water recovery distillation process. In both South African patents 98/11312 and 2001/9287 the separated organic layer from the overheads of the water recovery distillation process was routed to the recovery of the counter-solvent from the counter-solvent layer formed by the liquid-liquid extraction process. In both South African patents 98/11312 and 2001/9287 the counter-solvent recovery was achieved by subjecting the counter-solvent layer to a counter-solvent recovery distillation process to yield recovered counter-solvent (usually as an overheads product) and neutral oils and/or tar bases (usually as a bottoms product). The recovered counter-solvent was then recycled to the liquid-liquid extraction step and the neutral oils and/or tar bases were they routed to a neutral oils and/or tar bases processing facility.
It has now been established that in certain cases the overheads product of the water recovery distillation process contains significant concentration of phenols which form part of the organic layer which is separated from the aqueous layer of the overheads product of the water recovery distillation process (particularly when operating the extract stripper or water recovery distillation under pressure). In the prior art processes of South African patents 98/11312 and 2001/9287 these phenols were lost since they were routed to the counter-solvent recovery process where the phenols ended up with the neutral oils and/or tar bases which were routed to the neutral oils and/or tar bases processing facility.
It is an object of the present invention to at least reduce the above disadvantage.
Furthermore, as stated above in respect of South African patent 2001/19287, after or during the liquid-liquid extraction step the immiscible solvent and counter-solvent layers are separated from each other and the phenols are recovered from the solvent layer (as described above) while the neutral oils and/or nitrogen bases are recovered from the counter-solvent layer. In South African patents 98/11312 and 2001/9287 the counter-solvent layer was subjected to a counter-solvent recovery distillation process to yield recovered counter-solvent (as an overheads product) and neutral oils and/or tar bases (as a bottoms product). The recovered counter-solvent was then recycled to the liquid-liquid extraction step and the neutral oils and/or tar bases were then routed to a neutral oils and/or tar bases processing facility. South African patent 98/4165 describes a similar process for recovering the counter-solvent and in that case the neutral oil (as bottom product) was either used as a feedstock for napthalenes or it was combusted for fuel value.
It has now been established that to obtain optimum conditions for optimum separation of phenols from neutral oils during the liquid-liquid extraction the ratio of solvent to counter-solvent is such that some of the solvent is transferred to the counter-solvent layer. During the counter-solvent recovery distillation process the solvent forms part of the bottom product with the neutral oils and/or tar bases. Since the bottoms product was in the past forwarded to a neutral oils and/or tar base processing facility, or was used as a naphtalenes feedstock, or was combusted as fuel, the solvent contained therein was lost to the process.
It is an object of the present invention to reduce the above disadvantage.